MESA, Ariz. -- On the day Starlin Castro began his rehab for a strained hamstring, his sub at shortstop, Javier Baez, hit his first spring home run.

Is there room on the Cubs for both shortstops?

"Yeah," Castro said Tuesday, adding that he told Baez: " 'If it helps the team win, and you're there, I'm there, whatever. I don't care.' "

The Cubs have insisted Baez will open the 2014 season as Triple-A Iowa's shortstop, and Castro's injury is not serious enough to sideline him for Opening Day. But each at-bat by Baez makes Cubs fans giddy and prompts more questions to manager Rick Renteria about the team's top prospect.

"Obviously for us, the idea of Baez is in the future," Renteria said Tuesday. "I'm glad Starlin feels that way [that there's room for both] and I'm sure the feeling is mutual. They both respect each other immensely. Somewhere down the road when that happens, I'm sure it'll be nice to see."

Castro, who strained his right hamstring on Sunday sliding into second base, had an MRI on his leg, which showed a little bit of fluid, but nothing serious. The Cubs expect Castro to be sidelined seven to 10 days, which is less time than the shortstop needed last season when he missed two weeks with a strained left hamstring.

"I don't want to rush myself," Castro said. "I have more experience with [the injury]. This year is not as bad as last year. This year, it's a little less than last year. I have a lot of experience for what we'll do."

He also has access to the Cubs' hydrotherapy pool at their new spring complex, which should help his rehab. In the meantime, he'll help mentor Baez.

"He can hit, no doubt about it," Castro said of Baez, who was the Cubs' No. 1 pick in 2011. "He's a good player. Everybody knows that."

Could Baez be promoted this year?

"Maybe, I don't know," Castro said. "I don't make that decision. It would be good for us if he's there."

Baez belted 37 home runs last season combined at Class A Advanced Daytona and Double-A Tennessee, and was named the Cubs' Minor League Player of the Year.

"I think he has the bat speed to catch up to any fastball that anybody throws," Renteria said. "Once he starts seeing more breaking balls and offspeed pitches, and they start trying to manipulate the zone with him, do I think he'll ultimately be able to make that adjustment? Yes. Do I know if he can hit [Major League pitchers] now? I couldn't tell you that."

Castro is trying to ease the transition from the Minor Leagues to the big leagues.

"We're good friends and we talk a lot," Castro said. "Some people don't have a good relationship because they play the same position, but [not] me and him. That's what I tell him. 'Play hard. You'll be up there no matter what. Where -- I don't know where, but you'll be there because you have great talent and you play the right way.' "

"A lot of people talk about me and him," Baez said, "but we're really close and we look out for each other and we're just trying to play together.

"He helps me, and I help him when I can help him."

Everyone is looking out for Baez. After the fifth inning on Monday, Renteria called the young shortstop over and they chatted, ending the exchange with laughter and smiles. Renteria called it a "nice conversation."

"He was just explaining that when the catcher goes to the pitcher, go there and see what they're talking about if they change the signs with a man on second," Baez said. "That way I can let the infield know."

It's all part of the development process. Renteria likes what he's seen of Baez's defense, but says both the 21-year-old Baez and the 23-year-old Castro have elements of their game that need work.

"There are a lot of times when they don't finish coming through a ball," Renteria said of his young shortstops. "They'll catch balls and stay flat-footed and kind of throw flat-footed. A lot of errors come from that particular position. They both have strong arms and depend on that and like being able to show it. You can still fundamentally correctly get around a baseball a little more and consistently follow your throw to make sure what would be the quote-unquote routine play is continually made. Both of them are working on those kind of things right now."

The Cubs want Baez to focus on shortstop, but fans also may see him at second or third this spring. Castro is set at short.

"If he's there [in the big leagues], he has to play," Castro said.

The Cubs know that, too.

"As time goes on and Javy gains experience that we believe he needs to solidify himself as a more complete ballplayer at the Major League level, he still has to put in his time and gain some experience and some substance to his game," Renteria said, "because, quite frankly, you don't want an individual to make a jump and then have to go back again. You want him to come to stay."